This invention relates to a toner for developing electrostatic images which are formed by electrophotography, electrostatic printing, electrostatic recording or the like.
The development process of the electrostatic images comprises sucking charged fine particles by means of an electrostatic force and then depositing them on the surface of an electrostatic image support to thereby visualize the electrostatic images.
Concrete techniques of carrying out such a development process are a wet development where a liquid developing agent is employed which can be prepared by dispersing a pigment or a dyestuff into an insulating organic liquid, a dry development such as cascade development, a fur brush development, a magnetic brush development, an impression development or a powder cloud development where a powdery developing agent comprising a toner is used which can be prepared by dispersing a colorant such as carbon black into a binder comprising a naturally occurring or a synthetic resin.
The images visualized in the development process may be fixed intact on the support, but they are usually transferred to another support such as a transfer paper and are then fixed thereon. As apparent from the foregoing, the toner is subjected to not only the development process but also the subsequent processes, i.e. a transfer process and a fixing process; therefore, the toner is required to have excellent transfer and fixing properties in addition to good developing properties. Of these requirements, the fixing properties are most severe to satisfy, and research on the improvements in the fixing characteristics of the toner and their results has heretofore been published in a variety of litrature.
It is generally advantageous that a toner image formed in the development process or its transferred image be fixed by a heat fixing system and the heat fixing system include a non-contact fixing system such as an oven fixing and a contact heat fixing system such as a heat roller fixing. The contact heat fixing system advantageously has a high thermal efficiency and is suitable for a fixing step in a high-speed duplicating machine, because it permits a high-speed fixing operation, Further, since it permits a relatively low temperature heat source to be employed, the contact heat fixing system consumes less electric power; therefore, it can be contemplated to miniaturize the duplicating machine and to save energy. Furthermore, even if papers happen to stagnate in a fixing device, no ignition will occur, which fact is an additional advantage of the contact heat fixing system.
Although it is excellent in various points as described above, the contact heat fixing system has such serious problems as the occurrence of offset development. This is the phenomenon in which a part of the toner constituting an image is transferred to the surface of a heat roller at the time of the fixing operation and the partially transferred toner is transferred again to the next transfer paper or the like thereby soiling the paper. For the purpose of preventing this offset phenomenon, a variety of suggestions has heretofore been made and some of them have been put into practice. One such system comprises carrying out the fixing while an oily parting agent such as silicone oil is applied to the surface of the heat roller, and others thereof contemplate providing the toner itself with an offset preventing ability. The latter are excellent in that silicone oil applicator is not needed. Accordingly the structure of the fixing device can be simplified and any maintenance to replenish silicone oil or the like is not necessary.
The offset phenomenon tends to occur when the temperature of the heat roller is raised; therefore, if a minimum temperature (hereinafter referred to as the offset phenomenon occurring temperature), which the toner has and above which the offset phenomenon will occur, is at a higher level, the toner can be considered to be better in non-offset properties. However, in order to permit the toner to be fixed, the toner is required to be heated up to a temperature of its softening point or more. Accordingly, in the real heat roller fixing device, the temperature of the heat roller is set to a specified level within a fixable temperature section in a range between the softening point of the toner up to the offset occurring temperature.
In fact, the temperature of the heat roller cannot be completely uniformly maintained at a set temperature and there are other factors to be considered regarding the temperature. Therefore, a toner which has an extensive fixable temperature section and which can maintain the advantages of the contact heat fixing system is suitable.
As a binder for the toner, a styrene resin and a vinyl polymer have heretofore widely been used because they are easy to manufacture, low in cost and generally have preferred properties as the binder. With the toner including the vinyl polymer as the binder, it has been suggested to employ a high molecular weight polymer for the purpose of obtaining the non-offset properties. However, such a high molecular weight vinyl polymer has a high softening point. Accordingly, it is hard to procure an extensive fixable temperature section even though the offset occurring temperature is raised to a high level. In consequence, any suitable toner has not yet been prepared.
On the other hand, with regard to a polyester resin, since its low molecular weight resin can be obtained comparatively easily, toner having a low softening point can be prepared by employing such a resin as the binder. Further, as compared with the toner including the vinyl polymer as the binder, the toner including the polyester resin is better in "wetting" to such a support as the transfer paper when molten, with the result that the polyester resin toner advantageously permits the sufficient fixing at a lower temperature than occurs with the vinyl polymer toner which has a substantially similar softening point.
Moreover, judging from the fact that the toner is deposited on the electrostatic image support by virtue of an electrostatic attraction force and in certain cases it is further transferred to another support, it is required that the toner has a generally uniform and stable frictional chargeability. A usual way of providing the toner with the frictional chargeability is to incorporate a charge controlling agent such as a dyestuff into resin particles constituting the toner. Therefore, a severe condition of uniformly dispersing the charge controlling agent in the resin is additionally imposed on the manufacturing process of such a toner. However, since the polyester resin itself has a suitable frictional chargeability, such an incorporation of the charge controlling agent is unnecessary, or even if necessary, a lower amount will suffice. In short, if the polyester resin is employed as the binder, the preparation of the toner can be very highly easily accomplished.
However, the toner including the polyester resin as the binder usually has the drawback that its offset occurring temperature is low.
Further, when a styrene resin alone is employed as the binder, more problems requiring improvement are observed. Particularly, in order to raise the offset occurring temperature of the toner in which the styrene resin is used as the binder, a high molecular weight component incorporated into the binder resin of the toner is effective. However, if it is attempted to provide the toner with the sufficient non-offset characteristics by this manner, the softening point of the resin will be high, which fact will lead to a rise of the minimum temperature (hereinafter referred to as the minimum fixing temperature) necessary for the fixing of the toner, so that the advantages of the contact heat fixing system will be lost. It is, needless to say, desirable that the minimum fixing temperature below and in order to form visible images on both surfaces of one transfer paper sheet, which has been recently increasingly desired, the toner must be able to be fixed at a low temperature.
In view of this background, for the purpose of preparing the toner which has the non-offset characteristics and a low minimum fixing temperature, there have been developed techniques in which a ratio Mw/Mn of a weight average molecular weight Mw to a number average molecular weight Mn of the binder resin has been increased to raise a cohesive force of the toner at the time of its melting and another technique in which a wax having a low softening point and comprising a parting agent such as a polypropylene or a polyethylene wax is incorporated into the toner particles in order to lower the minimum fixing temperature.
In the former technique, however, when it is intended to obtain the sufficient non-offset characteristics, the minimum fixing temperature rises and when a particular binder resin is selected with the aim of inhibiting such a temperature rise, its glass transition temperature drops, so that the toner tends to cohere.
Accordingly, a combination of the former technique and the latter could be presumed; but when the technique wherein the low softening point-having wax is incorporated into the toner is employed, the powdery toner is poor in fluidity and developing and transfer characteristics are also lowered, so that good visible images cannot be formed. Additionally, such a toner tends to bring about cohesion. Further, in order to obtain an effect resulting from the addition of the wax, an amount proportion of the wax must be enhanced up to a relatively high level. If done, however, the wax component will adhere to a developing sleeve or an electrostatic charge support, thereby forming a coating film thereon, so that its function is disadvantageously obstructed.
For the improvement in a fluidity of the toner, adding a fluidity improver comprising a hydrophobic silica fine powder to the toner powder is effective; but in order to give the sufficient fluidity to the toner in which the low softening point wax is contained, it is necessary to add a great deal of the hydrophilic silica fine powder thereto. If such a powder is actually used, the surface of the electrostatic image support is damaged by the hydrophobic silica fine powder which is very hard and when the support is a light conductive sensitizer, its the problem is serious. Further, if cleaning the surface of the electrostatic image support is carried out by a rubber blade, the blade becomes violently worn. Furthermore, in an image forming device having a recycle and reuse system for returning to the developing device, there is toner which has been fed thereto for the development but which has not been utilized for the formation of the toner image are the fine particles of the hydrophobic silica become embedded in the surface portions of the toner particles to lower the fluidity of the toner, whereby the formed visible image is poor in quality.
The present invention has been achieved over the background of the aforementioned situations and its object is to provide a toner for development of electrostatic images which has sufficient non-offset characteristics, a low minimum fixing temperature, a high fluidity and less cohesiveness which and always permits stable forming of good visible images.